Documentation: xfs: Fix typo

In "Y+P" of this line, there are two non-ASCII characters(0xd9 0x8d)
following behind the 'Y'.  Shown as a small '=' under the '+' in VIM
and a '賺' in webpage[1].

I think it's a mistake and remove these strange characters.

[1]: https://www.kernel.org/doc/Documentation/filesystems/xfs-delayed-logging-design.txt



Signed-off-by: Shiyang Ruan <ruansy.fnst@cn.fujitsu.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>